Fix images overflowing on mobile by preserving aspect ratio

max-width: 100% alone lets img/picture height stay at intrinsic size,
so on narrow viewports images could still overflow vertically or look
squashed. Adding height: auto keeps the aspect ratio correct as width
scales down.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

resources/css/reset.css

@@ -56,6 +56,7 @@
   img,
   picture {
     max-width: 100%;
+    height: auto;
     display: block;
   }